1. Field of the Invention
The present invention relates to a grain-oriented electrical steel sheet having improved glass film properties and a low watt loss, and a process for producing the same.
2. Description of the Related Arts
Grain-oriented electrical steel sheet is mainly used for the cores of electrical appliances, such as transformers and power generators. For such usage, it is important that the grain-oriented electrical steel sheet have excellent magnetic properties such as the watt-loss characteristics and excitation characteristics, and excellent glass film properties. Usually, the grain-oriented electrical steel sheet is produced by the steps of hot-rolling a silicon-steel slab containing 4% or less of silicon, and if necessary, hot-coil annealing; cold-rolling once or twice or more with an intermediate annealing therebetween to obtain a cold-rolled sheet having a final sheet thickness; decarburization-annealing; applying an annealing separator mainly composed of MgO; finishing annealing to develop secondary recrystallized grains having a Goss texture; removing impurities such as S and N; forming a glass film; and finally, heat-flattening and treating with an insulating coating.
An improvement of the magnetic properties, particularly the watt loss, together with an improvement of the glass film has been investigated, and it is known, as shown in J. Appl. Phys. 38, (1967), pp. 1104 .about.1108, that a reduction in the sheet thickness and grain-refinement of a grain-oriented electrical steel sheet effectively reduces the watt loss. Reducing the sheet gauge is an effective method of reducing the watt loss, but the watt loss is increased due to an increase in the eddy current loss when the sheet thickness becomes less than a predetermined thickness. An improvement of the watt loss by grain-refinement is inherently limited by the secondary recrystallization phenomenon, which is utilized to attain a growth of grains having the Goss texture and enhance the orientation degree.
For an improvement of a glass film, Japanese Unexamined Patent Publication (Kokai) No. 50-71526, for example, describes the pickling of a grain-oriented electrical steel sheet, which was cold-rolled to a final thickness, in such a manner that 3 g/m.sup.2 or more of its surface layer is uniformly removed, thereby removing the surface deposits and a superficial part of the steel part thereof, and thus enabling a uniform progression in the decarburization reaction and the oxide-formation reaction. This, in turn, leads to a formation of an MgO-SiO.sub.2 series insulating film having an improved uniformity and adhesiveness after the decarburization annealing, application of an annealing separator, and finishing annealing.
Japanese Unexamined Patent Publication (Kokai) No. 57-101673 discloses that, after the decarburization annealing of a grain-oriented electrical steel strip cold-rolled to a final thickness and before the application of the annealing separator, such as MgO and the like, the surface of the steel strip is subjected to grinding or pickling so as to remove 0.025 to 0.5 g/m.sup.2 of the surface per one side, thereby removing the oxide film constituting the surface layer of a grain-oriented electrical steel sheet. Subsequently, the annealing separator is applied, and finishing annealing is carried out. The thus-formed glass film has a uniform, grey appearance, and an improved adhesiveness.
Japanese Unexamined Patent Publication (Kokai) No. 61-96082 proposes to grind and clean the surface of a steel sheet, without forming unevennesses, by a grinding means consisting of soft materials including a carborundum abrasive and an alundum abrasive, thereby enabling a uniform subscale of SiO.sub.2 to be formed during the decarburization annealing and a uniform and dense film to be formed during the finishing annealing.